Anode for Lithium Ion Secondary Battery, Method for Preparing the Same and Lithium Ion Secondary Battery Comprising the Same

ABSTRACT

The present disclosure relates to an anode for a secondary battery, a method of manufacturing the anode, and a lithium ion secondary battery including the anode. An anode includes an anode mixture layer on at least one surface of an anode current collector, with pores inside the anode mixture layer having a Z-tensor value of 0.33 or more. In addition, a method of manufacturing the anode and a lithium ion secondary battery including the anode are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/515,609 filed on Nov. 1, 2021, which claims priority to Korean PatentApplication No. 10-2020-0144518 filed Nov. 2, 2020, the disclosures ofwhich are hereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to an anode for a lithium ion secondarybattery, and a method for preparing the anode, and a lithium ionsecondary battery including the same.

Description of Related Art

In general, an anode for a lithium secondary battery is manufactured byapplying an anode mixture manufactured including an anode activematerial, a conductive agent, a binder and a solvent to an anode currentcollector, drying the same, and rolling the same. In this process, ananode active material having an anisotropic structure is mainly orientedin a direction, parallel to the anode current collector (horizontaldirection), and in a process in which a secondary battery is charged anddischarged, lithium ions will be moved internally through pores formedbetween the anode active materials in a horizontal direction.

However, when lithium ions move through the pores formed in thehorizontal direction, as loading of an electrode increases, a distanceto move to an inside of the electrode increases significantly, resultingin an increase in resistance in a charging process. In particular, whencharging is performed at a high C-rate, lithium salt (Li-plating) isformed on a surface of the electrode, and a cycle is repeated, resultingin a problem in which battery capacity decreases.

In this case, when an anode active material has an orientation in adirection, perpendicularly to the anode current collector (verticaldirection), the formed pores may also be formed in the verticaldirection, resulting in an effect of shortening a length of a passagethrough which lithium ions move internally. Thereby, resistance of thebattery can be lowered, and in particular, charging and dischargingefficiency at a high C-rate can be improved, thereby improving rapidcharging performance.

Therefore, in a process of applying an anode mixture to a surface of ananode current collector, a technique, in which a magnetic field isapplied to orient an anode active material vertically with respect to ananode current collector, and to control an XDR value of the anode activematerial in the finally obtained anode within a predetermined range, hasbeen proposed. However, since the XRD value according to orientation ofthe anode active material itself is not a value for a movement path oflithium ions, there is a limit that is not directly related to theperformance of the battery, and an alternative thereto is required.

SUMMARY OF THE INVENTION

The present disclosure is to improve battery performance by facilitatinginsertion and deintercalation of lithium ions, and is to provide ananode in which pores in an anode mixture layer are orientedperpendicularly to an anode current collector so as to directly improvebattery performance, and a lithium ion secondary battery including theanode.

The present disclosure is to provide an anode for a lithium ionsecondary battery that can directly determine battery performancethrough a degree of orientation of pores in an anode mixture layer, anda battery including the same.

Furthermore, an object of the present disclosure is to provide a methodfor aligning the pores in the anode mixture layer to be perpendicular tothe anode current collector.

An object of the present disclosure is to provide a method ofmanufacturing an anode for a lithium ion secondary battery to directlydetermine battery performance through a degree of orientation of poresin an anode mixture layer.

According to the present disclosure, an anode for a lithium ionsecondary battery is provided. The anode for a lithium ion secondarybattery includes an anode mixture layer on at least one surface of ananode current collector, and pores in the anode mixture layer have aZ-tensor value of 0.33 or more.

The anode mixture layer may include at least one selected from a groupconsisting of artificial graphite, natural graphite, and silicon as ananode active material.

The anode active material may be at least one selected from a groupconsisting of amorphous, plate-like, flake-like, spherical, and fibrousshapes.

The anode mixture layer may include 94 to 98% by weight of an anodeactive material, 0.1 to 3% by weight of a conductive agent, and 1.5 to3% by weight of a binder based on a total weight of the anode mixturelayer.

The anode mixture layer may have electrode density of 1.50 g/cc or moreon one side.

In addition, the present disclosure relates to a method for preparing ananode for a secondary battery, wherein the method includes operationsof: forming an anode mixture layer by coating an anode mixture includingan anode active material on at least one surface of an anode currentcollector (operation A); and changing orientation of the anode activematerial by applying a magnetic field to the anode mixture layer(operation B), wherein pores in the anode mixture may have a Z-tensorvalue of 0.33 or more.

The anode mixture preferably has viscosity in a range of 5,000 to 50,000cp at a temperature of 25° C. and a shear rate of 0.1 s⁻¹.

A magnetic field is preferably applied to the anode mixture layer for aperiod of 1 second or more and 30 seconds or less.

The magnetic field applied to the anode mixture layer preferably hasintensity of 1,000 Gauss or more and 25,000 Gauss or less.

The anode mixture may include 94 to 98% by weight of an anode activematerial, 0.1 to 3% by weight of a conductive agent, and 1.5 to 3% byweight of a binder based on a total weight of a solid content of theanode mixture.

The method for preparing the same may further include an operation ofdrying the anode mixture layer after operation B (operation C).

After operation B, the method for preparing the same may further includean operation of rolling the anode mixture layer (operation D), whereinthe rolling may be performed so that electrode density of the anodemixture layer on one side is 1.50 g/cc or more.

Furthermore, in the present disclosure, a lithium ion secondary batteryincluding an electrode assembly in which the anode as described aboveand a cathode including a cathode mixture layer on at least one surfaceof a cathode current collector are alternately stacked with a separatoras a boundary, and a battery case in which the electrode assembly isaccommodated and sealed, is provided.

The electrode density of the anode mixture layer on one side may be 1.50g/cc or more.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will be more clearly understood from the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a three-dimensional (3D) image of a structure of an anodemixture layer obtained using an X-ray microscope.

FIG. 2 is an image of a structure of pores within an anode mixture layerobtained by converting the 3D image of a structure of the anode mixturelayer obtained in FIG. 1.

FIG. 3 is a diagram schematically illustrating a concept of anorientation tensor for evaluation a degree of orientation of poreswithin an anode mixture layer as a 3D structure, and in FIG. 3, (a)illustrates a case of pores being randomly oriented (3-D random) in allaxial directions of X1, X2, and X3, (b) illustrates a case of poresbeing oriented on a plane formed by two axes of X1 and X2 (planarrandom), and (c) illustrates a case of pores being oriented and alignedin a direction of X1.

FIG. 4 is a graph illustrating changes in a capacity retention rateaccording to the number of cycles in the lithium ion secondary batteriesobtained in Examples 1 and 4 and Comparative Examples 1, 4, and 5.

DESCRIPTION OF THE INVENTION

The following detailed description is provided to assist the reader ingaining a comprehensive understanding of the methods, apparatuses,and/or systems described herein. However, various changes,modifications, and equivalents of the methods, apparatuses, and/orsystems described herein will be apparent to one of ordinary skill inthe art. The sequences of operations described herein are merelyexamples, and are not limited to those set forth herein, but may bechanged, as will be apparent to one of ordinary skill in the art, withthe exception of operations necessarily occurring in a certain order.Also, descriptions of functions and constructions that would be wellknown to one of ordinary skill in the art may be omitted for increasedclarity and conciseness.

The terminology used herein describes particular embodiments only, andthe present disclosure is not limited thereby. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “including”, “comprises,” and/or“comprising” when used in this specification, specify the presence ofstated features, integers, steps, operations, members, elements, and/orgroups thereof, but do not preclude the presence or addition of one ormore other features, integers, steps, operations, members, elements,and/or groups thereof.

Throughout the specification, it will be understood that when anelement, such as a layer, region or wafer (substrate), is referred to asbeing “on,” “connected to,” or “coupled to” another element, it may bedirectly “on,” “connected to,” or “coupled to” the other element orother elements intervening therebetween may be present. In contrast,when an element is referred to as being “directly on,” “directlyconnected to,” or “directly coupled to” another element, there may be noelements or layers intervening therebetween. Like numerals refer to likeelements throughout. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

The drawings may not be to scale, and the relative size, proportions,and depiction of elements in the drawings may be exaggerated forclarity, illustration, and convenience.

Hereinafter, exemplary embodiments will be described with reference tovarious examples. However, embodiments of the present disclosure may bemodified in various other forms, and the scope of the present disclosureis not limited to the embodiments described below.

In the present specification, when a part of a layer, film, region,plate, or the like is said to be “above” or “on” another part, whichincludes not only cases in which it is “directly on” another part, butalso a case in which another part is interposed therebetween.

The present inventors have completed the present disclosure, focusing onthe fact that a movement path of lithium ions is not a anode activematerial itself, but pores inside an anode mixture layer formed on ananode current collector.

That is, the present disclosure provides an anode for a secondarybattery having a Z-tensor having a specific value obtained by evaluatingan orientation tensor in each axial direction, after three-dimensionallyimaging the pores inside the anode mixture layer formed by applying amagnetic field thereto using an X-ray microscope. That is, it ispossible to provide a secondary battery having improvedcharging/discharging efficiency and rapid charging performance at a highrate by determining a degree of orientation of the electrode mixturelayer, a movement path of lithium ions.

In the present disclosure, a ‘vertical direction’ means oriented at 90°with respect to the anode current collector, but is not limited to 90° ,and for example, may mean oriented at 30° or more, more preferablyoriented at 45° or more, and most preferably oriented at 60° or more.

In addition, that the pores are oriented in a vertical direction doesnot mean that all the pores should be vertically oriented, it means thatthe pores oriented in the vertical direction exist predominantly, and inthe present disclosure, a degree to which the pores are oriented in avertical direction with respect to a current collector may be expressedas a Z-tensor.

The anode for a lithium secondary battery of the present disclosure isan anode for a lithium ion secondary battery including an anode mixturelayer on at least one surface of an anode current collector, wherein thepores within the anode mixture layer may have a Z-tensor value of 0.33or more.

For measurement of the Z-tensor value, the pores inside the electrodemay be imaged using an X-ray microscope for the cathode, the Z-tensorvalue may be derived from the the obtained image of pores.

More specifically, when an X-ray microscope is used for an interior ofan anode mixture layer, as shown in FIG. 1, a three-dimensionalstructure of the pores within the anode mixture layer can be obtained.Furthermore, it can be converted into a 3D image of the internal porestructure of the anode mixture layer as shown in FIG. 2, by 3D renderingfrom an image of the 3D structure inside the obtained anode mixturelayer. As an apparatus capable of obtaining such a 3-D image, a facilityincluding an X-ray source, a detector, and a lens capable of magnifyingthe source between the detectors may be used, and for example, thefacility may be a Zeiss's Xraida 520 Versa. In addition, a software forthe 3-D rendering may be GEODICT.

From the three-dimensional structure of the internal pores of theobtained anode mixture layer, a degree of orientation in each axialdirection of each pore may be expressed as an orientation tensor. Thatis, the three dimensional structure for each pore may be evaluated inthree axes of X, Y, and Z, and the sum of the tensors for the three axesin one object becomes ‘1’, and the concept of this was schematicallyshown in FIG. 3.

FIG. 3 illustrates a degree of orientation for any one object as anorientation tensor, in FIG. 3, (a) illustrates a case of being 3-Drandomly oriented, (b) illustrates a case of pores being oriented in aplane formed by two axes of X1 and X2, that is, a case of pores 3-Drandomly oriented (planar random) in one plane, and (c) illustrates acase of being oriented and aligned in an X1 axis direction.

The larger the orientation tensor value of a specific axis, the more itis oriented to the corresponding axis. That is, as the Z-tensor valueincreases, it can be evaluated that the orientation is developed in adirection of a Z-axis (corresponding to an X3 axis in FIG. 3).

Referring to FIG. 3, for example, when the X, Y, and Z-tensor values are0.33, respectively, random orientation is shown in (a) of FIG. 3, andwhen the Z-tensor value is shown to be greater than 0.33, indicatingthat the anode active material tends to be oriented in a Z-axisdirection, that is, perpendicularly to the anode current collector. Thatis, when the Z-tensor value is greater than 0.33, it may mean that poresoriented in the Z-axis direction in the anode mixture layer arepredominantly formed.

In some cases, when the orientation along a specific axis is developed,that is, when a specific orientation tensor value exceeds 0.33, theshape along the corresponding axis is developed and the length from anobject to the corresponding axis may be increased.

In the present disclosure, the Z-tensor value of the pores within theanode mixture layer preferably has a value of 0.33 or more. When theZ-tensor value of the pores or more, the orientation of the pores insidethe anode mixture layer are oriented in a vertical direction withrespect to the anode current collector, may be increased, therebyshortening a moving path of lithium ions. Therefore, insertion anddeintercalation of lithium ions may be easily performed in acharging/discharging process, and accordingly, charging/dischargingefficiency at a high rate can be improved to improve fast chargingperformance, and in addition, diffusion resistance of lithium ions intothe electrode may be reduced during the charging/discharging process,thereby suppressing formation of lithium salts (Li-plating) on a surfaceof the electrode may be suppressed.

In the present disclosure, in order to improve the orientation of thepores in the anode mixture layer in a vertical direction with respect toan anode current collector, a method of applying an anode mixture to theanode current collector, and applying a magnetic field thereto. Afterforming an anode mixture layer by applying the anode mixture to theanode current collector, the anode active material in the anode mixturelayer may be oriented in a vertical direction, and further, poresoriented in a direction perpendicular to the anode current collector maybe developed in the anode mixture layer.

In this case, the application of the magnetic field may be controlled bychecking a change in the orientation of the anode active material andthe pores according to the intensity of the magnetic field, theapplication time of the magnetic field, and the viscosity of the anodemixture.

The application of the magnetic field may be performed under theconditions that a magnetic field having intensity in a range of 1,000Gauss or more and 25,000 Gauss or less, for example, a magnetic fieldhaving intensity in a range of 2,000 Gauss or more, 15,000 Gauss orless, or 2,500 Gauss or more and 7,500 Gauss or less, is applied for 1second or more, 30 seconds or less, for example, 1 second or more and 10seconds or less.

In this case, the anode mixture preferably has viscosity of 5,000 cp ormore and 50,000 cp or less, based on measurement at a temperature of 25°C. and a shear rate of 0.1 s⁻¹. When the viscosity of the anode mixtureis less than 5,000 cp, it is advantageous in terms of self-alignment ofthe anode active material and may increase orientation of the pores, butthere may be a problem in that an active material in the slurry easilyprecipitates because the viscosity may be too low, and when theviscosity thereof exceeds 50,000 cp, it may be difficult to orient theanode active material and the pores by applying a magnetic field, andthere is a problem in that coating processability is deterioratedbecause the viscosity is too high. That is, as the anode slurry hasviscosity of 5,000 cp or more, 50,000 cp or less, and preferably 30,000cp at a temperature of 25° C. and a shear rate of 0.1 s⁻¹, an effect ofinhibiting precipitation of the active material and a self-alignmenteffect of the active material and the pores may be simultaneouslyobtained.

In addition, as the viscosity is lower within the aforementionedviscosity range, resistance to a flow of the anode mixture decrease evenif a magnetic field intensity and a magnetic field application time arethe same, and accordingly, it is easy to increase orientation in avertical direction of the anode active material with respect to thecurrent collector, such that it may be advantageous to manufacture thepores in the anode mixture layer to have a Z-tensor value of 0.33 ormore.

In the present disclosure, the anode mixture may include an anode activematerial, a conductive agent, a binder, and water as a solvent, and mayfurther include a thickener, and the like, if necessary.

As the anode active material, a carbon-based anode active material maybe used. The carbon-based anode active material is not particularlylimited, as long as it is commonly used in manufacturing an anode for alithium ion secondary battery, and may be suitably used in the presentdisclosure, but may be artificial graphite or a mixture of artificialgraphite and natural graphite. When a crystalline carbon-based material,artificial graphite or a mixture of artificial graphite and naturalgraphite, is used as the anode active material, crystlallographicproperties of particles are more developed as compared to the case ofusing an amorphous carbon-based active material, it is possible tofurther improve orientation properties of the carbon material in anelectrode plate with respect to an external magnetic field, therebyimproving orientation of the pores.

A form of the artificial graphite or natural graphite may be amorphous,plate-like, flake-like, spherical or fibrous, and may also be acombination of two or more thereof. In addition, when the artificialgraphite and natural graphite are mixed and used, a mixing ratio thereofmay be 70:30 to 95:5 by weight.

In addition, the anode active material may further include at least oneof a Si-based anode active material, and a Sn-based anode activematerial or a lithium vananium oxide anode active material, togetherwith the carbon-based anode active material. When the anode activematerial further includes the same, it may be included in a range of 1to 50 wt % based on a total weight of the anode active material.

The Si-based anode active material may be Si, a Si—C composite,SiO_(x)(0<x<2), and a Si-Q alloy, wherein Q is an element, other thanSi, selected from a group consisting of alkali metals, alkaline earthmetals, group 13 elements, group 14 elements, group 15 elements, group16 elements, transition metals, rare earth elements, and combinationsthereof, and specifically, may be selected from a group consisting ofMg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg,Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Jr, Pd, Pt, Cu, Ag, Au, Zn, Cd, B,Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinationsthereof.

The Sn-based anode active material may be Sn, SnO₂, a Sn—R alloy,wherein R is not Sn and Si, and is an element selected from a groupconsisting of alkali metals, alkaline earth metals, group 13 elements,group 14 elements, group 15 elements, group 16 elements, transitionmetals, rare earth elements, and combinations thereof, and specifically,may be selected from a group consisting of Ca, Sr, Ba, Ra, Sc, Y, Ti,Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os,Hs, Rh, Jr, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Ge, P, As,Sb, Bi, S, Se, Te, Po, and combinations thereof. In addition, at leastone of these and SiO₂ may be mixed and used.

The anode active material may be included in an amount of 94 to 98% byweight based on a solid content of the anode mixture.

In an embodiment, the anode mixture includes a binder. The binder servesto bind anode active material particles to each other, and also to bindthe anode active material an anode current collector. As the binder, anaqueous binder may be used.

The aqueous binder may include styrene butadiene rubber acrylatedstyrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylicrubber, butyl rubber, ethylene-propylene copolymer, polyepichlorohydrin,polyphosphazene, polyacrylonitrile, polystyrene,ethylene-propylene-diene copolymer, polyvinylpyridine, chlorosulfonatedpolyethylene, latex, a polyester resin, an acrylic resin, a phenolicresin, an epoxy resin, a polyvinyl alcohol resin, an acrylate-basedresin, or combinations thereof.

The content of the binder in the anode active material layer may be 1.5to 3 wt % based on a solid content of the anode mixture.

Together with the binder, a thickener may be further included to impartviscosity. The thickener may include a cellulose-based compound, forexample, carboxymethyl cellulose, hydroxypropyl methyl cellulose, methylcellulose, or alkali metal salts thereof may be used by mixing one ormore kinds thereof. As the alkali metal, Na, K or Li may be used. Thethickener may be 0.1 parts by weight to 3 parts by weight based on 100parts by weight of the anode active material.

The conductive agent is used to impart conductivity to an electrode, andmay be used without limitation as long as it is commonly used insecondary batteries, for example, may include a carbon-based materialsuch as natural graphite, artificial graphite, carbon black, acetyleneblack, Ketjen black, carbon fiber, or the like; a metal-based materialsuch as metal powders such as copper, nickel, aluminum, silver, and thelike, or metal fibers; a conductive polymer such as polyphenylenederivatives, or the like; or a conductive material including a mixturethereof.

The conductive agent may be used in an amount of 0.1 to 3% by weightbased on a solid weight of an anode mixture.

As the anode current collector, one selected from a group consisting ofcopper foil, nickel foil, stainless steel foil, titanium foil, stainlesssteel foil, titanium foil, nickel foam, copper foam, a polymer substratecoated with conductive metal, and combinations thereof may be used. Thethickness of the anode current collector is not particularly limited,and may be, for example, 5 to 30 μm.

As described above, by applying an anode mixture to at least one surfaceof an anode current collector, applying a magnetic field to orient ananode active material and pores, and then drying and rolling the same,an anode having an anode mixture layer formed on the anode currentcollector may be manufactured.

The drying process is for removing a solvent included in the anodemixture, and a drying means is not particularly limited, and aconventional drying means may be applied, for example, heating-dryingsuch as hot-air drying may be mentioned.

The drying process is not particularly limited, but may be performed forexample, within a range of 60 to 180° C., preferably within a range of70 to 150° C. for 20 to 300 seconds, for example, for 40 to 240 seconds,and for 60 to 200 seconds.

A rolling process may be performed after the drying process, and athickness or density of the anode mixture layer may be adjusted throughthe rolling process. Conventional methods such as a roll press methodand a flat plate press method can be used for the rolling treatment, andthe thickness of the anode mixture layer may be 20 μm or more and 120 μmor less per one side by the rolling process, for example, 40 μm or more,100 μm or less, or 60 μm or more and 80 μm or less.

Meanwhile, since the present disclosure may be suitably applied to anelectrode having high density having an anode mixture layer havingdensity of 1.5 g/cm² or more, and rolling may be performed to 1.5 g/cm²or more, and may be performed to, for example, 1.5 g/cm² or more, 2.2g/cm² or less, or 1.5 g/cm² or more, and 2.0 g/cm² or less.

The anode according to the present disclosure facilitates diffusion oflithium ions into an electrode through the anode active material and thepores, especially when the pores are oriented in a directionperpendicular to the anode current collector, thereby improvingcharging/discharging efficiency at high rate, and improving rapidcharging performance.

The anode including the pores with the developed vertical orientation ina vertical direction obtained by the present disclosure and a cathodemay be alternately stacked with a separator as a boundary, and theninserted and sealed in a battery case and an electrolyte may be injectedthereto to manufacture a lithium ion secondary battery.

Hereinafter, a cathode will be described in more detail. The cathode isnot particularly limited, but a cathode mixture layer is formed byapplying a cathode mixture to at least one surface of a cathode currentcollector, drying and rolling the same, and any cathode commonly used insecondary batteries can be suitably used in the present disclosure.

The cathode mixture may include a cathode active material, a binder, anda solvent, and if necessary, a conductive agent, and may also include athickener.

As the cathode active material, a compound capable of reversibleinsertion and deintercalation of lithium (a lithiated intercalationcompound) may be used. Specifically, at least one of complex oxides oflithium and metal selected from cobalt, manganese, nickel, andcombinations thereof may be used.

A more specific example may be a lithium transition metal compound(oxide) having a layered structure as represented by the general formulaLiMO₂, wherein M includes at least one of transition metal elements suchas Ni, Co, and Mn, and may further include another metal element or anon-metal element. The composite oxide, may be for example, a monolithiclithium transition metal composite oxide containing one type of thetransition metal element, a so-called binary lithium transition metalcomposite oxide containing two types of the above transition metalelement, and a ternary lithium transition metal composite oxidecontaining Ni, Co, and Mn as a constituent element, as a transitionmetal element, and as the composite oxide, a ternary lithium transitionmetal composite oxide such as Li(Ni_(1/3)Co_(1/3)Mn_(1/3))O₂ ispreferable.

In addition, as a lithium transition metal compound (oxide) representedby general formula Li₂MO₃, wherein M may include at least one oftransition metal elements such as Mn, Fe, Co, and may further includeanother metal element or a non-metal element, for example, Li₂MnO₃,Li₂PtO₃, and the like.

In addition, it may be a solid solution of LiMO₂ and Li₂MO₃, forexample, a solid solution represented by 0.5LiNiMnCoO₂-0.5Li₂MnO₃.

Furthermore, a material having a coating layer on a surface of thecathode active material may be used, or a mixture of the compound and acompound having a coating layer may be used. The coating layer mayinclude at least one coating element compound selected from a groupconsisting of oxide, hydroxide, oxyhydroxide, oxycarbonate, andhydroxycarbonate of a coating element. The compound constituting thesecoating layers may be amorphous or crystalline. As the coating elementincluded in the coating layer, Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge,Ga, B, As, Zr, or a mixture thereof may be used.

In the cathode, the cathode active material may be 90 to 98% by weightbased on a solid content of a cathode mixture.

The binder serves to bind cathode active material particles to eachother, and bind the cathode active material to a cathode currentcollector, and may be in an amount of 1.5 to 5% by weight based on thesolid content of the cathode mixture. The binder may be, for example,polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose,diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride,polyvinyl fluoride, a polymer including ethylene oxide, polyvinylPyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidenefluoride, polyethylene, polypropylene, styrene-butadiene rubber,acrylated styrene-butadiene rubber, an epoxy resin, nylon, and the like.

Together with the binder, a thickener may be further included to impartviscosity. The thickener may be the same as the thickener included inthe anode mixture, and may be included in an amount of 0.1 to 3 parts byweight based on 100 parts by weight of the cathode active material.

The conductive agent is used to impart conductivity to a cathode, andany electro conductive material commonly used in the cathode of thesecondary battery may be suitably used, and the conductive agent used inthe anode mixture may be used. The conductive agent may be used in anamount of 0.1 to 5% by weight based on a solid content of the cathodemixture.

The solvent may be an aqueous solvent such as water, as well as anon-aqueous solvent. The non-aqueous solvent may be used in the presentdisclosure as long as it is commonly used in the preparation of acathode mixture for a secondary battery, for example,N-methyl-2-pyrrolidone (NMP), but is not limited thereto.

As a cathode current collector, metal having good conductivity, forexample, aluminum, nickel, titanium, stainless steel, and the like, maybe used, and the cathode current collector may have various forms suchas a sheet type, a thin type, and a mesh type. The thickness of thecathode current collector is not particularly limited, and may be 5 to30 μm.

As described above, a cathode having a cathode mixture layer formed on acathode current collector may be manufactured by applying a cathodemixture to at least one surface of the cathode current collector, anddrying and rolling the same.

The drying and rolling process may be performed by the same method asthat of manufacturing the anode, and a detailed description thereof willbe omitted.

A separator interposed between the cathode and the anode may be a poroussheet or nonwoven fabric, and the like, and may be a multilayer film ofpolyethylene, polypropylene, polyvinylidene fluoride or two or morelayers thereof, a mixed multilayer film of two layers ofpolyethylene/polypropylene, a mixed multilayer film of three layers ofpolyethylene/polypropylene/polyethylene, a mixed multilayer film ofthree layers of polypropylene/polyethylene/polypropylene, and the like,and furthermore, the separator may be provided with a porousheat-resistant layer on one side or both sides of the porous sheet ornonwoven fabric, or the like. The separator is not particularly limited,but, for example, the separator may have a thickness of about 10 to 40μm.

The electrolyte includes a non-aqueous organic solvent and a lithiumsalt. The non-aqueous organic solvent serves as a medium through whichions involved in an electrochemical reaction of a battery can move, forexample, and the organic solvent may be carbonate-based, ester-based,ether-based, ketone-based, alcohol-based, or aprotic solvent. Thenon-aqueous organic solvent may be commonly used in lithium ionsecondary batteries, and the organic solvents may be used alone or incombination of one or more thereof.

The lithium salt is dissolved in an organic solvent, serves as a supplysource of lithium ions in the battery, enables a basic operation oflithium secondary batteries, and promotes movement of lithium ionsbetween the cathode and the anode. For example, as the lithium salt, oneor two or more elements selected from a group consisting of LiPF, LiBF₄,LiSbF₆, LiAsF₆, LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃,LiClO₄, LiAlO₂, LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (where,x and y are each independently an integer from 1 to 20), LiCl, and LiIand LiB(C₂O₄)₂(lithium bis(oxalato) borate: LiBOB) may be used. Aconcentration of the lithium salt is not particularly limited, but maybe used within a range of 0.1M to 2.0M.

The electrolyte may further include vinylene carbonate or an ethylenecarbonate-based compound as needed to improve battery life. That is, asthe anode according to the present disclosure, the secondary batteryincluding an anode for a lithium secondary battery in which orientationof the pores in the anode mixture layer is dominant in a Z-axis, thatis, the pores within the anode mixture layer have a Z-tensor value of0.33 or more, facilitates diffusion of lithium ions in the electrodeduring the charging and discharging process, thereby lowering theresistance of the battery, and in particular, improving charging anddischarging efficiency at a high rate, thereby improving lifespancharacteristics and rapid charging performance.

EXAMPLE

The electrolyte may further include a vinylene carbonate or an ethylenecarbonate-based compound as needed to improve battery life. That is, asthe anode according to the present disclosure, the secondary batteryincluding an anode for a lithium secondary battery in which orientationof the pores in the anode mixture layer is dominant in a Z-axis, thatis, the pores within the anode mixture layer have a Z-tensor value of0.33 or more, facilitates diffusion of lithium ions in the electrodeduring the charging and discharging process, thereby lowering theresistance of the battery, and in particular, improving charging anddischarging efficiency at a high rate, thereby improving cycle lifecharacteristics and rapid charging performance.

Examples 1 to 4 and Comparative Examples 1 to 5

An anode mixture was manufactured by mixing 89.3 wt % of artificialgraphite, 5 wt % of silicon oxide, 1.5 wt % of styrene butadiene rubber,and 1.2 wt % of carboxymethylcellulose 1.2 CNT in water. In this case,viscosity of the anode mixture is as shown in Table 1 at a temperatureof 25° C. and a shear rate of 0.1 s⁻¹.

The manufactured anode mixture was coated on top and bottom surfaces ofan anode current collector of Cu foil, and the anode current collectorwas passed between a pair of neodymium magnets in which a magnetic fieldof 4,000 Gauss was formed. In this case, by changing a moving speed ofthe anode current collector coated with the anode mixture, a time forwhich the magnetic field is applied as shown in Table 1 was adjusted,followed by drying and rolling to manufacture an anode. However, inComparative Example 1, a magnetic field was not applied.

The manufactured anode was evaluated for 3-D Z-tensor values of poresusing an X-ray microscope (Xraida 520 Versa, 3D rendering software:GEODICT, manufactured by Zeiss), and results thereof are shown in Table1.

A cathode mixture was manufactured by mixing 96% by weight of a cathodeactive material of Li(Ni_(0.8)Co_(0.1)Mn_(0.1))O₂, 2% by weight of a CNTconductive agent, and 2% by weight of a polyvinylidene fluoride binderin an N-methylpyrrolidone solvent. Using the manufactured anode mixture,Al foil was coated on both surfaces of the cathode current collector,and then dried and rolled to manufacture a cathode.

The manufactured anode and a cathode are alternately stacked with aseparator as a boundary, inserted into a pouch, sealed, and then a mixedsolvent of ethylene carbonate and diethyl carbonate (50:50 volumeratio), in which 1M LiPF₆ is dissolved (in a 50:50 volume ratio), wasinjected to manufacture a lithium ion secondary battery.

Charging (2.5 C) and discharging (0.3 C) at a high C-rate were repeatedfor each of the manufactured secondary batteries, and a capacityretention rate was measured at 100 cycles and 300 cycles, and resultsthereof are shown in Table 1. Furthermore, with respect to the lithiumion secondary batteries of Examples 1 and 3 and Comparative Examples 1,4, and 5, a change in a capacity retention rate according to the numberof cycles was shown in FIG. 4.

TABLE 1 Magnetic field Capacity retention rate(%) Viscosity of anodeapplied (2.5C charging/0.3C mixture (cp) (application?) discharging)Classification (25° C., 0.1 s⁻¹) time (sec.) Z-tensor 100 cycle 300cycle Comparative 29,290 0 (not applied) 0.24 80.4 <80.0 example 1Comparative 29,290 0.5 0.25 100.0 85.2 example 2 Example 1 29,290 1.00.34 100.1 98.2 Example 2 29,290 4.0 0.33 100.4 98.9 Example 3 29,2908.0 0.33 100.3 99.1 Example 4 11,500 4.0 0.42 100.7 99.5 Comparative54,130 4.0 0.30 100.0 90.2 example 3 Comparative 77,210 4.0 0.25 99.8<80.0 example 4 Comparative 77,210 8.0 0.25 100.0 85.2 example 5

As can be seen from Table 1, in Comparative Example 1 in which nomagnetic field is applied to the anode mixture having the same viscosityas 29,290 cp and in Comparative Example 2 in which a magnetic fieldapplication time was as short as 0.5 seconds, a Z-tensor value was aslow as 0.24 and 0.25, respectively. On the other hand, in Examples 1 to4 in which a magnetic field application time was 1 second or longer, aZ-tensor value of pores within the anode mixture layer was as high as0.33 or higher. In addition, it could be confirmed that Examples 1 to 4exhibit a high capacity retention rate of 98% or more for up to 300cycles.

Meanwhile, looking at the high rate charging/discharging cycle results,in a secondary battery of Comparative Examples 1 and 2 including theanode having a low Z-tensor value of 0.33 or less in the pores in theanode mixture layer, the capacitince retention rate at 300 cycles wereless than 80%, and about 85%, respectively, indicating that thecapacitince retention rate was significantly lowered to 95% or less.That is, in Examples 1 to 3, it was confirmed that the batteryperformance was improved as a degree of orientation of the pores in theanode mixture layer, a moving path of lithium ions, in a verticaldirection with respect to a current collector developer.

Furthermore, as can be seen from FIG. 4, in the secondary battery ofExample 1 including the anode according to the present disclosure, thecapacity retention rate is maintained to be substantially constantduring 300 cycles of charging and discharging, while in the secondarybattery of Comparative example 1 to which no magnetic field was applied,the capacity retention rate was abruptly decreased from the beginning ofthe charging/discharging cycle, and the capacity retention rate at 100cycles was reduced to a level of 80%. Therefore, no further change incapacity was confirmed after 100 cycles.

The above-described results may be explained by shortening a movingdistance of lithium ions, reducing diffusion resistance of lithium ionsin the electrode during a charging process, and suppressing generationof lithium salts during charging at high rate, as vertical orientationand the degree in which the pores in the anode mixture layer areoriented in a Z-axis direction are improved.

In particular, the anode of Example 4 manufactured using an anodemixture having low viscosity had a higher value than that in the anodeof Example 2 manufactured by applying a magnetic field for the sametime, and the anode of Example 3 manufactured by applying a magneticfield for a longer time, and the lithium ion secondary battery ofExample 4 had a capacity retention rate on a level of 99.5% at 300cycles, confirming that the capacity retention rate was very high ascompared to the lithium ion secondary batteries of Examples 2 and 3. Inaddition, as can be seen from FIG. 4, in the secondary battery ofExample 4 including the anode according to the present disclosure, itcould be seen that the capacity retention rate was maintained almostconstant during 300 cycles of charging and discharging.

As described above, the results of battery performance are due to adifference in Z-tensor values. Even though a magnetic field of the sameintensity is applied for a shorter time, the viscosity of the anodemixture is low, and the resistance to a flow of the anode mixture isreduced, so that the orientation in which the anode active material andthe pores in the anode mixture layer are oriented in a verticaldirection with respect to the current collector may be further improved.

On the other hand, when the anodes of Comparative Examples 3 to 5manufactured using an anode mixture having a viscosity of 50,000 cp ormore were vertically oriented by applying a magnetic field of the sameintensity as in Example 4 for the same time, while a Z-tensor value wasless than 0.33, it was evaluated that orientation in which the pores ofthe anode mixture layer were oriented in a vertical direction withrespect to the anode current collector was not sufficiently developed.

It was confirmed that the lithium ion secondary batteries of ComparativeExamples 3 to 5 including such an anode also significantly deterioratedto a level of 90.2% or less in a capacity retention rate at 300 cycles.In addition, as can be seen from FIG. 4, in the second batteries ofComparative Examples 4 and 5, the capacity retention rate rapidlydecreased after 100 cycles. Comparative Example 4 showed a capacityretention rate of 85% or less at 200 cycles, and did not confirm achange in additional capacity, but was expected to have a capacitinceretention rate of less than 80% at 300 cycles.

Meanwhile, comparing Comparative Examples 4 and 5, a Z-tensor value wasnot affected even if a magnetic field application time is increasedwhile using the same mixture having the same viscosity, a result inwhich the Z-tensor value is not affected was shown.

Examples 5 to 6 and Comparative Examples 6 to 7

An anode was manufactured in the same manner as in Example 1, exceptthat viscosity of the anode mixture, and a magnetic field applicationtime were adjusted as shown in Table 2, and a Z-tensor value wasevaluated and results thereof are shown in Table 2.

In addition, in order to confirm a degree of orientation of an anodeactive material with respect to a surface of an anode current collectorfor the manufactured anode, XRD was measured, and results thereof areshown in Table 2.

Furthermore, after manufacturing a cathode and a lithium ion secondarybattery in the same manner as in Example 1, a capacity retention ratewas measured in the same manner as in Example 1 for each of themanufactured secondary batteries, and results thereof was shown in Table2.

TABLE 2 Capacity retention Viscosity of rate(%) anode mixture Magneticfield (2.5C charge/0.3C (cp) application discharge) Classification (25°C., 0.1 s⁻¹) time(Sec.) I(110)/I(002) Z-tensor 100 cycle 300 cycleComparative 11,340 0(not 0.15 0.30 100 86.2 Example 6 applied) Example 511,340 2 0.17 0.48 100 99.5 Comparative 12,440 0(not applied) 0.17 0.3299 85.9 Example 7 Example 6 12,440 2 0.78 0.42 100 98.8

When I(110)/I(002) is 0.5 (%) or more, it indicates that the anodeactive material is vertically oriented with respect to the anode currentcollector, and when I(110)/I(002)is closer to 0 (%), it indicates thatthe anode active material is horizontally oriented.

Comparing Example 5 and Comparative Example 6 in Table 2, a value ofI(110)/I(002) is 0.15% before self-alignment and 0.17% afterself-alignment. All of the corresponding values are 0.5% or less, whichis a value to an extent that an anode active material oriented in avertical direction with respect to the anode current collector, is notobserved despite application of a magnetic field.

However, compared to the anode of Comparative Example 6, the Z-tensorvalue of the anode of Example 5 increased by 0.18, indicating that thepores in the anode mixture layer developed in a direction,perpendicularly to an anode current collector by application of amagnetic field. An increase in orientation of the pores by theapplication of the magnetic field may not be confirmed by an XRDanalysis indicating that a degree of orientation of the anode activematerial, but may be confirmed from an evaluation of the Z-tensor value.

In this case, the anode of Example 5 has a Z-tensor value of 0.48,indicating that the pores oriented in a vertical direction of the anodecurrent collector are predominantly present in the anode mixture layer,and the secondary battery including the anode has a capacity retentionrate of 99.5%, indicating a significantly higher capacity retention ratecompared to Comparative Example 6.

Meanwhile, comparing Example 6 and Comparative Example 7 of Table 2, inthe case of I(110)/I(002), a ratio before self-alignment was 0.17%,whereas a ratio after self-alignment was applied, which can be seen thatthe value of I(110)/I(002) is changed by 0.5% or more.

In addition, the Z-tensor value of the anode of Example 6 increased by0.1 with respect to the anode of Comparative Example 7, indicating thatpores developed in a vertical direction of the anode current collectorby the application of a magnetic field. In this case, the anode ofExample 6 has a Z-tensor value of 0.42, indicating that the poresoriented in a vertical direction of the anode current collector arepredominantly present in the anode mixture layer, and the secondarybattery including such an anode had a capacity retention rate of 98.8%,which was significantly higher than that of Comparative Example 7.

From the results of Examples 5 and 6 and Comparative Examples 6 and 7 asdescribed above, when a change in the degree of orientation of the anodeactive material is shown as I(110)/I(002) derived from XDR measurement,as a magnetic field is applied, from results obtained by analyzing achange in an XDR value and a change in a Z-tensor value, it can be seenthat a value of I(110)/I(002) is not always directly related to batteryperformance.

On the other hand, when the pores oriented in the vertical direction ofthe anode current collector are developed in the anode mixture layer bythe application of a magnetic field, a movement path of lithium ionsmight be shortened, thereby improving the battery performance. That is,it could be seen that the Z-tensor value related to the orientation ofthe pores within the anode mixture layer, a passage for lithium ions,rather than the orientation of the anode active material in the anodemixture layer, is a characteristic directly related to the batteryperformance.

As set forth above, in the anode for a lithium ion secondary batteryaccording to the present disclosure, orientation in which pores withinthe anode mixture layer are oriented in a vertical direction withrespect to the anode current collector may be increased, therebyfacilitating insertion and deintercalation of lithium ions, therebyimproving charging/discharging efficiency of and rapid chargingperformance of the lithium ion secondary battery at a high rate.

According to the method for manufacturing an anode for a lithium ionsecondary battery according to the present disclosure, orientation inwhich pores within the anode mixture layer are oriented in a verticaldirection with respect to an anode current collector may be increased,and thus, an anode having improved charging and discharging efficiencyand rapid charging performance of the lithium ion secondary battery at ahigh rate may be manufactured.

While this disclosure includes specific examples, it will be apparent toone of ordinary skill in the art that various changes in form anddetails may be made in these examples without departing from the spiritand scope of the claims and their equivalents. The examples describedherein are to be considered in a descriptive sense only, and not forpurposes of limitation. Descriptions of features or aspects in eachexample are to be considered as being applicable to similar features oraspects in other examples. Suitable results may be achieved if thedescribed techniques are performed to have a different order, and/or ifcomponents in a described system, architecture, device, or circuit arecombined in a different manner, and/or replaced or supplemented by othercomponents or their equivalents. Therefore, the scope of the disclosureis defined not by the detailed description, but by the claims and theirequivalents, and all variations within the scope of the claims and theirequivalents are to be construed as being included in the disclosure.

What is claimed is:
 1. An anode for a lithium ion secondary battery,comprising: an anode mixture layer including an anode active materialand a binder on at least one surface of an anode current collector,wherein pores inside the anode mixture layer have a Z-tensor value of0.33 or more, and the anode active material of the anode mixture layerhas an I(110)/I(002) value of less than 0.50.
 2. The anode for a lithiumion secondary battery of claim 1, wherein the anode mixture layer haselectrode density of 1.50 g/cm² or more on one side.
 3. The anode for alithium ion secondary battery of claim 1, wherein the anode mixturelayer has electrode density of 1.50 g/cm² or more and 2.2 g/cm² or lesson one side.
 4. The anode for a lithium ion secondary battery of claim1, wherein the anode mixture layer has electrode density of 1.50 g/cm²or more and 2.0 g/cm² or less on one side.
 5. The anode for a lithiumion secondary battery of claim 1, wherein the anode mixture layercomprises 94 to 98% by weight of an anode active material, 0.1 to 3% byweight of a conductive agent, and 1.5 to 3% by weight of a binder basedon a total weight of the anode mixture layer.
 6. The anode for a lithiumion secondary battery of claim 1, wherein the anode active material isartificial graphite, natural graphite, or a mixture thereof.
 7. Theanode for a lithium ion secondary battery of claim 6, wherein the anodeactive material is at least one selected from a group consisting ofamorphous, plate-like, flake-like, spherical, and fibrous shapes.
 8. Theanode for a lithium ion secondary battery of claim 6, wherein the anodeactive material further comprises at least one selected from a groupconsisting of a silicon-based anode active material, a Sn-based anodeactive material, and a vanadium oxide.
 9. The anode for a lithium ionsecondary battery of claim 1, wherein the anode active materialcomprises artificial graphite and natural graphite, and the artificialgraphite and natural graphite are mixed in a weight ratio of 70 to 95:5to
 30. 10. The anode for a lithium ion secondary battery of claim 9,wherein the anode active material further comprises at least oneselected from a group consisting of a silicon-based anode activematerial, a Sn-based anode active material, and a vanadium oxide.
 11. Asecondary battery, comprising: a cathode; and the anode of claim 1.